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<title>GDB&rsquo;s Obsolete Annotations: Limitations</title>

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<a name="Limitations"></a>
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<p>
Next: <a href="Migrating-to-GDB_002fMI.html#Migrating-to-GDB_002fMI" accesskey="n" rel="next">Migrating to GDB/MI</a>, Previous: <a href="Annotations-Overview.html#Annotations-Overview" accesskey="p" rel="prev">Annotations Overview</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
</div>
<hr>
<a name="Limitations-of-the-Annotation-Interface"></a>
<h2 class="chapter">2 Limitations of the Annotation Interface</h2>

<p>The level two annotations mechanism is known to have a number of
technical and architectural limitations.  As a consequence, in 2001,
with the release of <small>GDB</small> 5.1 and the addition of <small>GDB/MI</small>,
the annotation interface was marked as deprecated.
</p>
<p>This chapter discusses the known problems.
</p>
<a name="Dependant-on-CLI-output"></a>
<h3 class="section">2.1 Dependant on <small>CLI</small> output</h3>

<p>The annotation interface works by interspersing markups with
<small>GDB</small> normal command-line interpreter output.  Unfortunately, this
makes the annotation client dependant on not just the annotations, but
also the <small>CLI</small> output.  This is because the client is forced to
assume that specific <small>GDB</small> commands provide specific information.
Any change to <small>GDB</small>&rsquo;s <small>CLI</small> output modifies or removes that
information and, consequently, likely breaks the client.
</p>
<p>Since the <small>GDB/MI</small> output is independent of the <small>CLI</small>, it does not
have this problem.
</p>
<a name="Scalability"></a>
<h3 class="section">2.2 Scalability</h3>

<p>The annotation interface relies on value annotations (see <a href="Value-Annotations.html#Value-Annotations">Value Annotations</a>) and the display mechanism as a way of obtaining up-to-date
value information.  These mechanisms are not scalable.
</p>
<p>In a graphical environment, where many values can be displayed
simultaneously, a serious performance problem occurs when the client
tries to first extract from <small>GDB</small>, and then re-display, all those
values.  The client should instead only request and update the values
that changed.
</p>
<p>The <small>GDB/MI</small> Variable Objects provide just that mechanism.
</p>
<a name="Correctness"></a>
<h3 class="section">2.3 Correctness</h3>

<p>The annotation interface assumes that a variable&rsquo;s value can only be
changed when the target is running.  This assumption is not correct.  A
single assignment to a single variable can result in the entire target,
and all displayed values, needing an update.
</p>
<p>The <small>GDB/MI</small> Variable Objects include a mechanism for efficiently
reporting such changes.
</p>
<a name="Reliability"></a>
<h3 class="section">2.4 Reliability</h3>

<p>The <small>GDB/MI</small> interface includes a dedicated test directory
(<samp>gdb/gdb.mi</samp>), and any addition or fix to <small>GDB/MI</small> must include
testsuite changes.
</p>
<a name="Maintainability"></a>
<h3 class="section">2.5 Maintainability</h3>

<p>The annotation mechanism was implemented by interspersing <small>CLI</small> print
statements with various annotations.  As a consequence, any <small>CLI</small>
output change can alter the annotation output.
</p>
<p>Since the <small>GDB/MI</small> output is independent of the <small>CLI</small>, and the
<small>GDB/MI</small> is increasingly implemented independent of the <small>CLI</small>
code, its long term maintenance is much easier.
</p>
<hr>
<div class="header">
<p>
Next: <a href="Migrating-to-GDB_002fMI.html#Migrating-to-GDB_002fMI" accesskey="n" rel="next">Migrating to GDB/MI</a>, Previous: <a href="Annotations-Overview.html#Annotations-Overview" accesskey="p" rel="prev">Annotations Overview</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
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